Heat treating processes can be carried out in furnaces of various designs. A work chamber is present in furnaces of these types wherein the heat treating process is carried out. In this work chamber the work temperature is raised to that required for accomplishing the desired treating process. The most common method of heating a furnace is the combustion process, that is, combusting an air-fuel mixture. The heat released from the combustion process heats the work in the furnace to the necessary operating temperature. In order to qualify as a heat treating furnace, the temperature in the furnace must be at least 1400.degree. F. The furnace combustion heating process can be divided into two general heating system classifications, the first being the direct fired heating system. In this system, combustion occurs directly in the furnace work area. Impingement of the combustion flame on the work may or may not be desirable; however, in either instance, the products of combustion directly contact the work. Heating of the work is accomplished by conduction, convection, and radiation. The types of materials heat treated in a directfired furnace do not require an atmosphere other than that which results from the combustion process.
The second classification of furnace heating system is the indirect system involving the use of indirect fired heaters and radiant tube heaters. In this classification of furnace heating systems, the flames from the combustion process and the products of combustion are isolated from the work in the furnace work chamber. In a radiant tube heat treating furnace, for example, combustion occurs within tubes which are located in the furnace work chamber or in other areas. As a result of the combustion, the radiant tubes are heated to the required operating temperature. The radiant tubes, by radiation and convection, heat the work in the work chamber to the necessary operating temperature. The products of combustion for an indirect fired furnace heating system do not come in contact with the work but rather are contained inside the radiant tube heater or indirect fired heater outside the furnace work chamber and are exhausted to the atmosphere. As a result of the isolation of the products of combustion from the work chamber, indirect fired heat treating furnaces afford the user the capability of choosing a controlled furnace atmosphere which will impart desired metallurgical characteristics to the work. The present invention involves this type of furnace heating system.
Current industry practice for obtaining a furnace atmosphere involves combustion of an air-fuel mixture in a gas generator to form a gas of controlled specific composition. Some popular types of gas generators are endothermic, exothermic, and endothermic/exothermic. These gas generators are installed at a location remote to the furnace. The gas from the generator which will be introduced into the heat treating furnace as the furnace atmosphere, is transported from the gas generator through a piping system to the furnace, into which it is introduced as the furnace atmosphere.
In recent times there has been increasing emphasis on energy conservation. There is a need to be more effective and efficient in the use of fossil fuels in the heat treating industry.
As is apparent from the above discussion, a heat treating operation employing a heat treating furnace heated by an indirect furnace heating system and using a furnace atmosphere obtained from a gas generator, has two distinct points at which an air-fuel mixture is consumed. The first point of air-fuel consumption is in the furnace heating system and the second point of fuel consumption is in the gas generator. Present furnace heating systems operate only to utilize the heat resulting from the combustion process to heat the work in the furnace. The products of combustion from the furnace heating system are regarded as waste products and are exhausted to the atmosphere. Gas generators employ the combustion process to obtain a gas of specific composition. The heat liberated or absorbed in a gas generator is a waste product.
A common type of indirect fired heat treating furnace is the radiant tube type furnace. A discussion of the combustion system used with this type of furnace will provide an understanding of combustion principles which generally apply to all indirect fired furnaces.
The combustion system for a radiant tube heat treating furnace comprises an air blower, a burner, and a radiant tube. The combustion process begins with air being drawn into the air blower through an inlet on the air blower. The blower raises the pressure of the air to a level in excess of atmospheric air pressure. This elevated air pressure is the motive force for the combustion system. The air, at an elevated pressure, moves from the blower through the piping system to the burner. At the burner, the air is mixed with fuel in a controlled ratio and ignition occurs. The combustion process occurs within the radiant tube. The positive pressure of the combustion system which originates from the blower is the motive force pushing the products of combustion through the radiant tube and exhausting them to atmosphere. Radiant tube burners may have a primary and secondary air adjustment for the purpose of controlling the length of the flame in the radiant tube. By adjusting the primary to secondary air ratio and establishing a flame throughout the entire length of the radiant tube, one can achieve uniform heat release over the entire length of the radiant tube.
It is the practice in the heat treating industry to fire indirect heating systems with excess air, that is, more air than is required for complete combustion. In general, heat treating furnaces fire with excess air.
The products of combustion from the firing of an indirect heating system with excess air contain carbon dioxide, water vapor, and oxygen. These components of the products of combustion from the firing of radiant tube in general are not present in a controlled heat treating atmosphere. Therefore in the current industry heat treating practices when a protective furnace atmosphere is required to carry out a metallurgical process a controlled atmosphere is introduced into the furnace which contains at most very low levels of the above gases. Some practices are carried out where the controlled atmosphere contains none of the above gases.
The discussion in the General Description of the Present Invention infra will explain how combustion products from radiant tubes can be purified as necessary, that is, removing from the products of combustion carbon dioxide, water vapor, oxygen, or other constituents to levels which will allow the furnace combustion system to function as a heat source and the source of furnace atmosphere.
A metal treating furnace is characterized by the presence of a closed work area in which work is inserted to be heated to the temperature necessary to perform the metallurgical process. Heating by convection and radiation is used to heat the work set forth in the enclosed area and the heat is achieved by using electric heating elements or by the firing of radiant tubes. When fired radiant tubes are used as the source of heat, the combustion products are conventionally vented to the atmosphere and an atmosphere of very specific and controlled composition taken from a source external to the furnace is introduced into the heated work area in order to protect the work from undesirable metallurgical reaction such as oxidation, or to impart specific metallurgical characteristics to the work, for example, carburizing or carbonitriding. At present the source of the atmosphere or carrier gas is an atmospheric generator making use of gases such as endothermic, exothermic, or exothermic/endothermic or industrial gases.